1. Field of the Invention
This invention relates to improvements in the art of positioning a sensor with respect to a data track carried by a recording medium. This invention particularly relates, though not exclusively so, to an improved tracking servo for precisely positioning a playback head with respect to a magnetic track prerecorded on a magnetic disk.
2. Description Relative to the Prior Art
Small magnetic disks have found wide use in recent years for data storage in computer applications. Before playing back the data carried by such a disk, a tracking servo accurately positions a playback head opposite a selected magnetic track. The head is typically mounted on a movable head carriage and the carriage is coupled to a servomotor. Given the need for highly accurate movement, a stepper motor is usually chosen for the servomotor because of its inherent precision. (See "High Density Magnetic Recording on a Mini Flexible Disk Drive," by D. Brar, IEEE Trans. on Magnetics, Vol. Mag.-17, No. 4, July 1981, pp. 1423-1425). The normal operation of a stepper motor consists of discrete angular movements of predetermined, and mostly uniform, magnitude. These movements are produced by a control signal consisting of a specially generated pulse train. As the Brar article relates, the angular movement of the stepper motor is mechanically transmitted to the head carriage by means such as a band positioner, a cam, or a lead screw.
An electronic still camera has been proposed which uses a small magnetic disk as its memory device (see "Electronic Still Camera," by Kihara, N. et al. Journal of Applied Photographic Engineering, Vol. 9, No. 5, October 1983, 159-163). A companion player is envisioned which, when a recorded disk is inserted into it, converts the signals recorded on the disk into a television signal. For a tracking servo for such a disk player, it is appropriate to borrow from the computer disk art, that is, to use the concept of a head carriage driven by a stepper motor. (For example, see "The Electronic Still Camera a New Concept in Photography, "Kihara, N. et al, IEEE Trans. on Consumer Electronics, Vol. CE-28, No. 3, August 1982, pp. 325-330, which shows in FIG. 15 a head carriage connected by a lead screw to a stepper motor.)
The control signal for the stepper motor can be provided by "hard wired" logic but, being essentially digital, it can easily be provided directly from a microprocessor (as shown in the Kihara article and in the Guide to Selecting and Controlling Step Motors, by Warner Electric Brake and Clutch Co., 1979, especially chapter 8 thereof). A microprocessor gives the tracking servo (of which the stepper motor is part) the ability to analyze, interpret and modify commands by use of its built-in memory as well as to make changes as a result of feedback information. Many complex functions can be carried out in the software of the microprocessor. For example, the control pulses can be arbitrarily spaced so as to accomplish a complex ramping function just before stopping the stepper motor (i.e., before sending the last pulse), pulse spacing can be varied with load, and so on.
While such servo capability sounds attractive, there are some problems with such a complex system. Being directed to the ordinary consumer, the disk player should include every economy that is practicable. Stepper motors are ordinarily complex and costly, but nonetheless used because they are thought to be essential. A related problem is the extent to which the stepper motor involves the microprocessor. Despite the substantial capability of a microprocessor, its intelligence is harnessed to the moment-by-moment requirements of the stepper motor while the head is being moved from track to track. That is, the stepper motor is constantly in need of intelligent input--in the form of correctly spaced pulses--from the moment the head leaves the previous track until it arrives at the next track. Otherwise the motor--and the head therewith--would stop mid-track. In effect, a sophisticated servomotor--the stepper motor--ties down the microprocessor to the task of generating a train of pulses, one after the other. It would be better if the respective roles could be reversed, that is, the sophistication of the microprocessor could be used to reduce the sophistication required in the servomotor. As the player ordinarily carries a microprocessor for other purposes, use of it to simplify the tracking servo would be a desirable bonus.